Transistor-based circuits, such as, for example, power amplifiers, are sensitive to variations in temperature and chip manufacturing process. The transconductance (gm) of a transconductance circuit (such as, for example, a transistor) in a transistor-based circuit may vary by up to around 15% as the temperature of the circuit is varied from −40° C. to 100° C. The transconductance may vary by up to 30% due to variations in chip manufacturing process, which is referred to hereinafter as “process”.
FIG. 1 shows a power amplifier 100. The power amplifier 100 includes a transconductance circuit 102 that produces an output current iout from an input voltage vin. A bias current ibias for the transconductance circuit 102 controls the transconductance gm of the transconductance circuit 102. A load impedance 104 is connected between the output of the transconductance circuit 102 and a power supply voltage Vdd. The voltage at the output of the transconductance circuit 102 comprises the output voltage vout. The gain of the power amplifier is given by gmZL, where ZL is the effective impedance of the load 104 at the frequency of operation.
The transconductance of the transconductance circuit 102 varies with temperature and process as indicated above. Furthermore, the impedance of the load 104 tends to decrease as the temperature increases. These changes cause the gain of the power amplifier 100 to change as the temperature changes, which is undesirable where the gain of the power amplifier 100 is required to be substantially constant.
It is an object of embodiments of the invention to at least mitigate one or more of the problems of the prior art.